1. Field of the Invention
The present invention relates to an LC filter, and more particularly, to a high frequency laminated LC filter.
2. Description of the Related Art
In general, a circuit structure of a band pass filter having three or more stages that transmits a signal having a specific frequency is shown in FIG. 8. The band pass filter is a three-stage band pass filter having LC resonators Q1 to Q3 of the first to third stages. These LC resonators Q1 to Q3 are such that the adjacent resonators are electrically coupled by the coupling capacitors Cs1, Cs2.
FIGS. 9 to 11 show conventional laminated band pass filter configurations used in the circuit structure described above. LC filter 1 illustrates a conventional filter used for LC resonators Q1 to Q3 of the first to the third stages. The laminate body 21 is constructed by stacking up the square shaped ceramic sheets 2. LC resonators Q1 to Q3 of the circuit shown in FIG. 8 include LC filters as shown in FIG. 9.
The inductors L1 to L3 of the LC resonators Q1 to Q3, respectively, include inductor patterns 3, 4, 5 as shown in FIG. 9. The inductor patterns 3, 4, 5 have the same pattern widths. The capacitors C1 to C3 of the LC resonators Q1 to Q3, respectively, are defined by the capacitor patterns 9, 10, 11, and leading edges 6, 7, 8 of the inductor patterns 3, 4, 5 that are disposed opposite to these capacitor patterns 9, 10, 11. The above-described LC resonators Q1 to Q3 are electrically connected by the coupling capacitors Cs1, Cs2 that are defined by the capacitor patterns 9 to 11 and the coupling capacitor patterns 12, 13 that are disposed opposite these capacitor patterns 9 to 11. These LC resonators Q1 and Q3 are capacitive-coupled to the capacitor pattern 14 to provide an input, and to the capacitor pattern 15 to provide an output, respectively. The shielding patterns 16a, 16b are arranged to sandwich these patterns 3 to 5, 9 to 15.
In the laminated body 21, an input terminal electrode 26, an output terminal electrode 27 and shielding terminal electrodes 28, 29 as shown in FIG. 10 are provided. The capacitor pattern 14 defining the input is connected to the input terminal electrode 26, and the capacitor pattern 15 defining the output is connected to the output terminal electrode 27. The lead portions of the inductor patterns 3, 4, 5 and one end portion of the shielding pattern 16a, 16b are connected to the shielding terminal electrode 28. The lead portions of the capacitor patterns 9 to 11 and the other end portion of the shielding pattern 16a, 16b are connected to the shielding terminal electrode 29.
In general, the inductor pattern 4 that defines the LC resonator Q2 at the second stage which located at the center is such that the magnetic field concentration at the pattern edge is larger, than the magnetic field concentration of the inductor patterns 3, 5 that define the LC resonators Q1, Q3 at the first and third stages which are located at the ends. Accordingly, a current density flowing through the inductor pattern 4 becomes larger than a current density flowing through the inductor patterns 3, 5, and thus, the current density produced is not uniform. As a result, an LC filter having this construction produces a poor Q characteristic.
In order to overcome the problems described above, preferred embodiments of the present invention provide a laminated LC filter having an excellent Q characteristic.
A laminated LC filter of a preferred embodiment of the present invention includes a laminated body including a plurality of insulation layers, a plurality of inductor patterns, and a plurality of capacitor patterns which are stacked on each other, and at least three LC resonators having a plurality of inductors that are defined by inductor patterns, and a plurality of capacitors arranged such that the capacitor patterns are disposed opposite to the inductor patterns inside of the laminated body, wherein a filter including at least three stages is constructed by connecting at least three of the LC resonators. The pattern widths of the inductor patterns defining the LC resonators at locations other than both ends thereof are wider than the pattern widths of the inductor patterns defining the LC resonators at both end locations. Further, the pattern widths of the inductor patterns defining the LC resonators are reduced at the end portions thereof.
Each of the inductors of the respective LC resonators has a multiplex structure in which two or more of the inductor patterns having substantially identical shapes are laminated via insulation layers.
The reduced pattern widths of the inductor patterns of the LC resonators at the end portions greatly reduce the magnetic field at the edges of the inductor patterns.
Moreover, the multiplex structure of the inductor greatly reduces the magnetic field generated in the vicinity of the inductor. As a result, the magnetic field at the edges of the inductor patterns is greatly decreased.